Micro mirror unit and method of making the same

ABSTRACT

A micro mirror unit includes a moving part carrying a mirror portion, a frame and torsion bars connecting the moving part to the frame. The moving part, the frame and the torsion bars are formed integral from a material substrate. The frame includes a portion thicker than the moving part.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 10/327,855, filed Dec. 26,2002, which is based on Japanese Application No. 2002-170291 filed Jun.11, 2002 now U.S. Pat. No. 6,817,725.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a micro mirror unit and a method ofmaking it. The micro mirror unit is an element incorporated e.g. in anoptical switching device which switches optical paths between aplurality of optical fibers, or in an optical disc drive which recordsdata onto an optical disc and/or reproduces data recorded on it.

2. Description of the Related Art

In recent years, optical communications technology is utilized widely ina variety of fields. In the optical communications, optical fibers serveas a medium through which optical signals are passed. When the opticalsignal passing through a given optical fiber is switched to anotheroptical fiber, so-called optical switching devices are used in general.In order to achieve high quality optical communications, the opticalswitching device must have high capacity, high speed and highreliability in switching action. In view of these, micro mirror unitsmanufactured by micro-machining technology is attracting attention as aswitching element to be incorporated in the optical switching device.The micro mirror units enable the switching operation without convertingoptical signals into electric signals between the optical paths on theinput side and the output side of the optical switching device. This isadvantageous to achieving the desired characteristics mentioned above.

Micro mirror units are disclosed e.g. in Japanese Patent Laid-Open No.4-343318 and No. 11-52278. Further, optical switching devices which usemicro mirror units manufactured by micro-machining technologies aredisclosed in the article “MEMS Components for WDM Transmission Systems”(Optical Fiber Communication [OFC] 2002, pp.89–90 etc.

FIG. 21 outlines an ordinary optical switching device 500. The opticalswitching device 500 includes a pair of micro mirror arrays 501, 502, aninput fiber array 503, an output fiber array 504, and a plurality ofmicro lenses 505, 506. The input fiber array 503 includes apredetermined number of input fibers 503 a. The micro mirror array 501is provided with the same plurality of micro mirror units 501 a eachcorresponding to one of the input fibers 503 a. Likewise, the outputfiber array 504 includes a predetermined number of input fibers 504 a.The micro mirror array 502 is provided with the same plurality of micromirror units 502 a each corresponding to one of the output fibers 504 a.Each of the micro mirror units 501 a, 502 a has a mirror surface forreflection of light. The orientation of the mirror surface iscontrollable. Each of the micro lenses 505 faces an end of acorresponding input fiber 503 a. Likewise, each of the micro lenses 506faces an end of a corresponding output fiber 504 a.

In transmitting optical signals, lights L1 coming out of the outputfibers 503 a pass through the corresponding micro lenses 505respectively, thereby becoming parallel to each other and proceeding tothe micro mirror array 501. The lights L1 reflect on their correspondingmicro mirror units 501 a respectively, thereby deflected toward themicro mirror array 502. At this point, the mirror surfaces of the micromirror units 501 a are oriented, in advance, in predetermined directionsso as to direct the lights L1 to enter their respective desired micromirror units 502 a. Then, the lights L1 are reflected on the micromirror units 502 a, and thereby deflected toward the output fiber array504. At this point, the mirror surfaces of the micro mirror units 502 aare oriented, in advance, in predetermined directions so as to directthe lights L1 into their respective desired output fibers 504 a.

As described, according to the optical switching device 500, the lightsL1 coming out of the input fibers 503 a reach the desired output fibers504 a due to the deflection by the micro mirror arrays 501, 502. Inother words, a given input fiber 503 a is connected with an output fiber504 a in a one-to-one relationship. With this arrangement, byappropriately changing deflection angles of the micro mirror units 501a, 502 a, switching can be performed and the lights L1 can be deflectedinto different output fibers 504 a.

FIG. 22 outlines another ordinary optical switching device 600. Theoptical switching device 600 includes a micro mirror array 601, a fixedmirror 602, an input-output fiber array 603, and a plurality of microlenses 604. The input-output fiber array 603 includes a predeterminednumber of input fibers 603 a and a predetermined number of output fibers603 b. The micro mirror array 601 includes the same plurality of micromirror units 601 a each corresponding to one of the fibers 603 a, 603 b.Each of the micro mirror units 601 a has a mirror surface for reflectionof light and orientation of the mirror surfaces is controllable. Each ofthe micro lenses 604 faces an end of a corresponding one of the fibers603 a, 603 b.

In transmitting optical signals, light L2 coming out of the input fiber603 a passes through the corresponding micro lens 604 and is directedtoward the micro mirror array 601. The light L2 is then reflected by acorresponding first micro mirror unit 601 a, and thereby deflectedtoward the fixed mirror 602, reflected by the fixed mirror 602, and thenenters a corresponding second micro mirror unit 601 a. At this point,the mirror surface of the first micro mirror unit 601 a is oriented, inadvance, in a predetermined direction so as to direct the light L2 toenter a predetermined one of the micro mirror units 601 a. Then, thelight L2 is reflected on the second micro mirror unit 601 a, and therebydeflected toward the input-output fiber array 603. At this point, themirror surface of the second micro mirror unit 601 a is oriented, inadvance, in a predetermined direction so as to direct the light L2 toenter a predetermined one of the output fibers 603 b.

As described, according to the optical switching device 600, the lightL2 coming out of the input fiber 603 a reaches the desired output fiber603 b due to the deflection by the micro mirror array 601 and the fixedmirror 602. In other words, a given input fiber 603 a is connected withan output fiber 603 b in a one-to-one relationship. With thisarrangement, by appropriately changing deflection angles of the firstand the second micro mirror units 601 a, switching can be performed andthe light L2 can be deflected into different output fibers 603 b.

FIG. 23 is a perspective view, partly unillustrated, of a portion of aconventional micro mirror unit 700 for incorporation in such devices asthe optical switching devices 500, 600. The micro mirror unit 700includes a mirror-formed portion 710 having an upper surface providedwith a mirror surface (not illustrated), an inner frame 720 and an outerframe 730 (partly unillustrated), each formed with come-like electrodesintegrally therewith. Specifically, the mirror-formed portion 710 hasends facing away from each other, and a pair of comb-like electrodes 710a, 710 b are formed respectively on these ends. In the inner frame 720 apair of comb-like electrodes 720 a, 720 b extend inwardly, correspondingto the comb-like electrodes 710 a, 710 b. Also, a pair of comb-likeelectrodes 720 c, 720 d extend outwardly. In the outer frame 730 a pairof comb-like electrodes 730 a, 730 b extend inwardly, corresponding tothe comb-like electrodes 720 c, 720 d. The mirror-formed portion 710 andthe inner frame 720 are connected with each other by a pair of torsionbars 740. The inner frame 720 and the outer frame 730 are connected witheach other by a pair of torsion bars 750. The pair of torsion bars 740provides a pivotal axis for the mirror-formed portion 710 to pivot withrespect to the inner frame 720. The pair of torsion bars 750 provides apivotal axis for the inner frame 720, as well as for the associatingmirror-formed portion 710, to pivot with respect to the outer frame 730.

With the above arrangement, in the micro mirror unit 700, a pair ofcomb-like electrodes, such as the comb-like electrode 710 a and thecomb-like electrode 720 a, are opposed closely to each other forgeneration of static electric force, and take positions as shown in FIG.24A, i.e. one of the electrode assuming a lower position and the otherassuming an upper position, when there is no voltage applied. When anelectric voltage is applied, as shown in FIG. 24B, the comb-likeelectrode 710 a is drawn toward the comb-like electrode 720 a, therebypivoting the mirror-formed portion 710. More specifically, in FIG. 23,when the comb-like electrode 710 a is given a positive charge whereasthe comb-like electrode 720 a is given a negative charge, themirror-formed portion 710 is pivoted in a direction M1 while twistingthe pair of torsion bars 740. On the other hand, when the comb-likeelectrode 720 c is given a positive charge whereas the comb-likeelectrode 730 a is given a negative charge, the inner frame 720 ispivoted in a direction M2 while twisting the pair of torsion bars 750.

As a conventional method, the micro mirror unit 700 can be made from anSOI (Silicon on Insulator) wafer which sandwiches an insulating layerbetween silicon layers. Specifically, first, as shown in FIG. 25A, awafer 800 is prepared which has a layered structure including a firstsilicon layer 801, a second silicon layer 802, and an insulating layer803 sandwiched between these silicon layers. Next, as shown in FIG. 25B,an anisotropic etching is performed to the first silicon layer 801 via apredetermined mask, to form the mirror-formed portion 710, torsion bars140, the comb-like electrode 710 a and other members to be formed on thefirst silicon layer 801. Next, as shown in FIG. 25C, an anisotropicetching is performed to the second silicon layer 802 via a predeterminedmask, to form the comb-like electrode 720 a and other members to beformed on the second silicon layer 802. Note that for the sake ofsimplification of the drawings, each of the FIG. 25A through FIG. 25Cgives only one sectional view, and each view includes a plurality ofsections taken at different locations in the wafer 800.

However, according to the conventional method of manufacture asdescribed above, the thickness of the wafer 800 is directly reflected onthe thickness of the micro mirror unit 700. Specifically, the thicknessof the micro mirror unit 700 is identical with the thickness of thewafer 800 which is used for the formation of the micro mirror unit. Forthis reason, according to the conventional method, the material wafer800 must have the same thickness as the thickness of the micro mirrorunit 700 to be manufactured. This means that if the micro mirror unit700 is to be thin, the wafer 800 of the same thinness must be used. Forexample, take a case of manufacturing a micro mirror unit 700 having amirror surface having a size of about 100 through 1000 μm. In view of amass of the entire moving part including the mirror-formed portion 710and the inner frame 720, the amount of movement of the moving part, thesize of the comb-like electrodes necessary for achieving the amount ofmovement, etc considered comprehensively, a desirable thickness of themoving part or the micro mirror unit 700 is determined. In thisparticular case the desirable thickness is 100 through 200 μm. As aresult, in order to manufacture the micro mirror unit 700 having such athickness, a wafer 800 having the thickness of 100 through 200 μm isused.

According to the conventional method, in order to manufacture a thinmicro mirror unit 700, a correspondingly thin wafer 800 must be used.This means that the greater diameter the wafer 800 has, the moredifficult to handle the wafer. For instance, take a case in which amicro mirror unit 700 is to be manufactured from an SOI wafer 800 havinga thickness of 200 μm and a diameter of 6 inches. Often, the wafer 800is broken in a midway of the manufacturing process. After formation ofthe predetermined structural members on the first silicon layer 801 asshown in FIG. 25B, strength of the wafer 800 is decreased, makingespecially difficult to handle the wafer during the machining on thesecond silicon layer 802. Thinness of the wafer 800 limits, as has beendescribed, the size of the flat surface of the wafer due to handlingdifficulties. Further, the limitation on the size of the flat surface ofthe wafer places a limitation on the manufacture of micro mirror arraychips. Specifically, when the micro mirror array chips are manufacturedby forming a plurality of micro mirror units in an array pattern on asingle substrate, the size of the array is limited.

FIG. 26 shows a micro mirror unit 700 mounted on a wiring substrate. Inthe figure, the micro mirror unit 700 shows a section taken on linesXXVI—XXVI in FIG. 23. According to the conventional micro mirror unit700 in FIG. 23, the moving part including the mirror-formed portion 710and the inner frame 720 has the same thickness as the outer frame 730.For this reason, when the micro mirror unit 700 is mounted onto thewiring substrate 810, in order to allow the moving part to moveproperly, a spacer 811 must be provided as shown in FIG. 26 between thewiring substrate 810 and the outer frame 730. By providing the spacer811 having a sufficient thickness between the micro mirror unit 700 andthe wiring substrate 810, it becomes possible to avoid a situation thatthe moving part makes contact to the wiring substrate 810 to becomeunable to move. In view of a mounting process of the micro mirror unit700 onto the wiring substrate 810, it is not efficient to provide thespacer 811 separately.

SUMMARY OF THE INVENTION

The present invention has been proposed under the circumstancesdescribed above. It is therefore an object of the present invention toprovide a micro mirror unit capable of reducing the limitation on thesize of the flat surface of the wafer used for the manufacture. Anotherobject of the present invention is to provide a method of making such amicro mirror unit.

According to a first aspect of the present invention, there is provideda micro mirror unit comprising: a moving part including a mirrorportion; a frame; and a torsion bar connecting the moving part to theframe. The moving part, the frame and the torsion bar are formedintegral from a common material substrate. The frame includes a portionthicker than the moving part.

With the above arrangement, the limitation on the size of the materialsubstrate, or the wafer, used for manufacturing the micro mirror unit isreduced. The micro mirror unit according to the first aspect of thepresent invention includes a frame which has a portion thicker than themoving part. Therefore, even if the mass of the entire moving part, theamount of movement of the moving part, the size of the comb-likeelectrodes necessary for achieving the amount of movement and so onrequire the moving part to have a first thickness as thin as 100 through200 μm for example, it is still possible to use a wafer having a secondthickness thicker than the first thickness, in the manufacture of themicro mirror unit. When using such a wafer, the second thickness ismaintained in a predetermined or larger area of the frame throughoutsteps for forming necessary members of the element, whereby the strengthof the wafer can be maintained. As a result, it becomes possible toappropriately prevent the wafer from being destroyed, in themanufacturing process of the micro mirror unit.

As described, the micro mirror unit according to the first aspect of thepresent invention includes a frame which has a portion thicker than themoving part. This means that the frame extends beyond the moving portionat least on one side thickness-wise of the element. Therefore, if theframe extends sufficiently on the side away from the mirror surface ofthe moving part, it becomes possible to mount the micro mirror unitdirectly onto a wiring substrate via the frame. This is because theframe extending sufficiently provides appropriate space between themoving part and the wiring substrate, and as a result, the movement ofthe moving part is not hindered by the wiring substrate. On the otherhand, if the frame extends sufficiently on the same side as is themirror surface of the moving part, it becomes possible to bond atransparent cover such as a glass plate directly onto the micro mirrorunit to protect the mirror surface. This is because the frame extendingsufficiently provides appropriate space between the moving part and thetransparent cover, and as a result, the movement of the moving part isnot hindered by the transparent cover.

As described, according to the micro mirror unit offered by the firstaspect of the present invention, it is possible to reduce the limitationon the size of the flat surface of the wafer used for the manufacture.Further, it becomes possible to appropriately bond adjacent members suchas a wiring substrate and a transparent cover without using spacersprepared separately.

According to a second aspect of the present invention, there is providedanother micro mirror unit comprising a moving part, a frame and atorsion bar connecting the moving part to the frame. The moving part,the frame and the torsion bar are formed integral from a materialsubstrate having a layered structure including an intermediate layer andsilicone layers sandwiching the intermediate layer.

The moving part includes: a first intermediate portion originating fromthe intermediate layer; a first structural member held in contact withthe first intermediate portion and provided with a mirror portion; and asecond structural member held in contact with the first intermediateportion on a side opposite to the first structural member.

The frame includes: a second intermediate portion originating from theintermediate layer; a third structural member held in contact with thesecond intermediate portion on a same side as the first structuralmember; and a fourth structural member held in contact with the secondintermediate portion on a same side as the second structural member, and

The fourth structural member extends beyond the second structural memberin a layering direction of the layered structure.

An micro mirror unit having such an arrangement can also reduce thelimitation on the size of the flat surface of the wafer used for themanufacture as described for the first aspect. Further, again asdescribed for the first aspect, it is possible to appropriately bondadjacent members such as a wiring substrate without using separatespacers. A preferred embodiment of the micro mirror unit according tothe second aspect further comprises a wiring substrate bonded to thefourth structural member.

Preferably, the micro mirror unit may further comprise a wiringsubstrate bonded to the fourth structural member. Also, the thirdstructural member may extend beyond the first structural member in thelayering direction.

According to a third aspect of the present invention, there is provideda micro mirror unit comprising a moving part, a frame and a torsion barconnecting the moving part to the frame. The moving part, the frame andthe torsion bar are formed integral from a common material substratehaving a layered structure including an intermediate layer and siliconelayers sandwiching the intermediate layer.

The moving part includes: a first intermediate portion originating fromthe intermediate layer; a first structural member held in contact withthe first intermediate portion and provided with a mirror portion; and asecond structural member held in contact with the first intermediateportion on a side opposite to the first structural member.

The frame includes: a second intermediate portion originating from theintermediate layer; a third structural member held in contact with thesecond intermediate portion on a same side as the first structuralmember; and a fourth structural member held in contact with the secondintermediate portion on a same side as the second structural member.

The third structural member extends beyond the first structural memberin a layering direction of the layered structure.

Preferably, the micro mirror unit may further comprise a transparentcover bonded to the third structural member.

Preferably, in the respective micro mirror units described above, themoving part may include a first comb-like electrode, and the frame mayinclude a second comb-like electrode for operation of the moving part bystatic electric force generated between the first and the secondcomb-like electrodes.

Preferably, the first comb-like electrode may be formed in the firststructural member, and the second comb-like electrode may be formed inthe fourth structural member at a portion contacting the secondintermediate portion.

Preferably, in the respective micro mirror units described above, themoving part may include: a relay frame connected to the frame via thetorsion bar; a mirror-formed portion spaced from the relay frame; and arelay bar connecting the relay frame to the mirror-formed portion, therelay bar extending in a direction across a direction in which thetorsion bar extends.

In the above case, the mirror-formed portion may include a thirdcomb-like electrode, and the relay frame may include a fourth comb-likeelectrode for operation of the mirror-formed portion by static electricforce generated between the third and the fourth comb-like electrodes.The third comb-like electrode may be formed in the first structuralmember, while the fourth comb-like electrode may be formed in the secondstructural member.

According to a fourth aspect of the present invention, there is provideda method for making a micro mirror unit provided with a moving part, aframe and a torsion bar. The method includes the steps of:

-   -   performing first etching to a material substrate in a thickness        direction of the substrate by using a first masking pattern and        a second masking pattern, the first masking pattern being        arranged to mask a region of the substrate that is to become at        least a part of the frame, the second masking pattern being        provided with a portion for masking a region of the substrate        that is to become the moving part;

removing the second masking pattern; and

performing second etching to the material substrate by using the firstmasking pattern.

Preferably, the first etching may be performed midway in the thicknessdirection of the substrate, the second etching being performed topenetrate the material substrate so that at least the moving part isformed.

Preferably, the first etching may be performed until the materialsubstrate is penetrated, the second etching being performed midway inthe thickness direction of the substrate so that at least the movingpart is formed.

According to a fifth aspect of the present invention, there is provideda method for making a micro mirror unit from a material substrate thatincludes a first silicon layer, a second silicon layer and anintermediate layer sandwiched between these silicon layers. The micromirror unit to be produced includes a moving part, a frame and a torsionbar. The method includes the steps of:

-   -   performing first etching to the first silicon layer of the        material substrate by using a first masking pattern and a second        masking pattern, the first masking pattern being arranged to        mask a region of the first silicon layer that is to become at        least a part of the frame, the second masking pattern including        a portion for masking a region of the first silicon layer that        is to become the moving part;

removing the second masking pattern; and

performing second etching to the first silicon layer by using the firstmasking pattern.

Preferably, the first etching may be performed midway in a thicknessdirection of the first silicon layer, the second etching being performeduntil the intermediate layer is reached.

Preferably, the first etching may be performed until the intermediatelayer is reached, and the second etching may be performed midway in athickness direction of the first silicon layer.

Preferably, the second masking pattern may further include a portion formasking a region of the first silicon layer that is to become acomb-like electrode in the frame.

According to a sixth aspect of the present invention, there is provideda method for making a micro mirror unit by using a first materialsubstrate including a first silicon layer, a second silicon layer and anintermediate layer sandwiched between these silicon layers, the micromirror unit including a moving part, a frame and a torsion bar. Themethod includes the steps of:

forming a first masking pattern including a portion for masking a regionof the first silicon layer that is to become the moving part;

making a second material substrate incorporating the first maskingpattern, by bonding a third silicon layer to a surface of the firstsilicon layer upon which the first masking pattern is formed;

performing first etching to the third silicon layer by using a secondmasking pattern including a portion for masking at least a part of theframe, the first etching being continued until the first silicon layeris reached; and

performing second etching to the first silicon layer exposed by thefirst etching, the second etching being performed by using the firstmasking pattern until the intermediate layer is reached.

Preferably, the first masking pattern may further include a portion formasking a region to become a comb-like electrode formed in the frame.

According to a seventh aspect of the present invention, there isprovided a method for making a micro mirror unit that includes a movingpart, a frame provided with a comb-like electrode and a torsion barconnecting the moving part to the frame. The method includes the stepsof:

performing first etching to a first silicon layer prepared as a firstmaterial substrate, the first etching being performed by using a firstmasking pattern including a portion to mask a region of the firstmaterial substrate that is to become the comb-like electrode, the firstetching being continued until the etching reaches a depth correspondingto a thickness of the comb-like electrode;

making a second material substrate that includes the first materialsubstrate, an intermediate layer held in contact with the first materialsubstrate, and a second silicon layer held in contact with theintermediate layer;

performing second etching to the first silicon layer by using a secondmasking pattern and a third masking pattern, the second masking patternincluding a portion to mask a region to become at least a part of theframe, the third masking pattern including a portion to mask a region tobecome the moving part and the comb-like electrode, the second etchingbeing continued until the etching reaches a midway portion of the firstsilicon layer;

removing the third masking pattern; and

performing third etching to the first silicon layer by using the secondmasking pattern until the comb-like electrode is reached.

According to an eighth aspect of the present invention, there isprovided a method for making a micro mirror unit by using a firstmaterial substrate including a first silicon layer, a second siliconlayer and an intermediate layer sandwiched between these silicon layers,the first silicon layer incorporating a torsion bar held in contact withthe intermediate layer, the micro mirror unit including a moving part, aframe and the torsion bar. The method includes the steps of:

forming a first masking pattern on the first silicon layer, the firstmasking pattern including a portion to mask a region to become themoving part;

making a second material substrate incorporating the first maskingpattern, by bonding a third silicon layer to a surface of the firstsilicon layer upon which the first masking pattern is formed;

performing first etching to the third silicon layer by using a secondmasking pattern including a portion to mask a region to become at leasta part of the frame, the etching being continued until the first maskingpattern is exposed; and

performing second etching to the first silicon layer by using the firstmasking pattern until the intermediate layer is reached.

The methods according to the fourth through the eighth aspects of thepresent invention enable manufacture of the micro mirror units accordingto the first through the third aspects of the present invention.Therefore, according to the methods offered by the fourth through theeighth aspects, it is possible to reduce the limitation on the size ofthe flat surface of the wafer used for the manufacture. Further, it ispossible to appropriately bond adjacent members to the manufacturedelement without using separate spacers.

Other features and advantages of the present invention will becomeapparent from the detailed description given below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a micro mirror unit according to a firstembodiment of the present invention;

FIG. 2 is a sectional view of the micro mirror unit taken in lines II—IIin FIG. 1;

FIG. 3 is a sectional view of the micro mirror unit taken in linesIII—III in FIG. 1;

FIG. 4 is a sectional view of the micro mirror unit taken in lines VI—VIin FIG. 1;

FIG. 5 shows a state in which the micro mirror unit in FIG. 1 is inoperation;

FIGS. 6A–6D show steps of a method of manufacturing the micro mirrorunit in FIG. 1;

FIGS. 7A–7D show steps following those of FIG. 6;

FIGS. 8A–8C show steps following those of FIG. 7;

FIGS. 9A–9D show steps of another method of manufacturing the micromirror unit in FIG. 1;

FIGS. 10A–10D show steps following those of FIG. 9;

FIGS. 11A–11D show steps of another method of manufacturing the micromirror unit in FIG. 1;

FIGS. 12A–12D show steps following those of FIG. 11;

FIGS. 13A–13D show steps of another method of manufacturing the micromirror unit in FIG. 1;

FIGS. 14A–14D show steps following those of FIG. 13;

FIG. 15 is a perspective view showing a micro mirror unit according to asecond embodiment of the present invention;

FIG. 16 is a sectional view taken in lines XVI—XVI in FIG. 15;

FIG. 17 shows the micro mirror unit of FIG. 15 mounted on a wiringsubstrate with a transparent cover attached;

FIGS. 18A–18C show steps of a method of manufacturing the micro mirrorunit in FIG. 15;

FIGS. 19A–19C show steps following those of FIG. 18;

FIGS. 20A–20C show steps following those of FIG. 19;

FIG. 21 is a schematic view showing a conventional optical switchingdevice;

FIG. 22 is a schematic view showing another conventional opticalswitching device;

FIG. 23 is a perspective view showing a conventional micro mirror unitprovided with comb-like electrodes.

FIGS. 24A–24B show the arrangement of comb-like electrodes operating ina pair;

FIGS. 25A–25C show steps of a method of manufacturing the conventionalmicro mirror unit in FIG. 23; and

FIG. 26 shows a state in which the micro mirror unit in FIG. 23 is inoperation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 is a perspective view of a micro mirror unit X1 according to afirst embodiment of the present invention.

FIG. 2 is a sectional view taken in lines II—II in FIG. 1.

FIG. 3 is a sectional view taken in lines III—III in FIG. 1, and FIG. 4is a sectional view taken in lines VI—VI in FIG. 1.

As shown in FIG. 1, the micro mirror unit X1 includes a mirror-formedportion 110, an inner frame 120 surrounding it, an outer frame 130surrounding the inner frame 120, a pair of torsion bars 140 connectingthe mirror-formed portion 110 with the inner frame 120 and a pair oftorsion bars 150 connecting the inner frame 120 with the outer frame130. The pair of torsion bars 140 provides a pivotal axis A1 for themirror-formed portion 110 to pivot with respect to the inner frame 120.The pair of torsion bars 150 provides a pivotal axis A2 for the innerframe 120, as well as the associating mirror-formed portion 110, topivot with respect to the outer frame 130. According to the presentembodiment, the pivotal axis A1 and the pivotal axis A2 are generallyperpendicular to each other. The micro mirror unit X1 is a single piecestructure made of electrically conductive material, except for itsmirror surface 111 and insulating layer 160 to be described later. Theelectrically conductive material is provided by e.g. silicon andpoly-silicon doped with an n-type impurity such as P and As or with ap-type impurity such as B.

The mirror-formed portion 110 has an upper surface formed with a thinfilm of mirror surface 111. Further, the mirror-formed portion 110 hastwo side surfaces facing away from each other and formed with comb-likeelectrodes 110 a, 110 b respectively.

The inner frame 120, which will be understood more clearly by referringto all of the FIG. 1 through FIG. 4, has a layered structure includingan inner frame main portion 121, a pair of electrode bases 122 and aninsulating layer 160 placed between them. The inner frame main portion121 and the electrode bases 122 are electrically separated by theinsulating layer 160. The pair of electrode bases 122 are formedrespectively with inwardly extending comb-like electrodes 122 a, 122 b.The inner frame main portion 121 has, as integral parts therewith,outwardly extending comb-like electrodes 121 a, 121 b. As shown clearlyin FIG. 2, the comb-like electrodes 122 a, 122 b are below the comb-likeelectrodes 110 a, 110 b of the mirror-formed portion 110. The comb-likeelectrodes 110 a, 110 b and 122 a, 122 b are positioned so as not tointerfere with each other when the mirror-formed portion 110 pivots, ina pattern shown e.g. for the comb-like electrode 110 a and the comb-likeelectrode 122 a in FIG. 4, i.e. their teeth are staggered each other.

As clearly shown in FIG. 3, the pair of torsion bars 140 are eachthinner than the mirror-formed portion 110, and are connected to themirror-formed portion 110 as well as to the inner frame main portion121.

As clearly shown in FIG. 2, the outer frame 130 has a layered structureincluding a first outer frame 131, a second outer frame 132 and aninsulating layer 160 between them. The first outer frame 131 and thesecond outer frame 132 are electrically separated by the insulatinglayer 160. As clearly shown in FIG. 3, the second outer frame 132 isformed, as integral parts thereof, with inwardly extending comb-likeelectrodes 132 a, 132 b. The comb-like electrodes 132 a, 132 b are belowthe comb-like electrodes 121 a, 121 b respectively of the inner framemain portion 121. The comb-like electrodes 121 a, 121 b and 132 a, 132 bare positioned in a staggered pattern so as not to interfere with eachother when the inner frame 120 pivots. As clearly shown in FIG. 2through FIG. 4, the second outer frame 132 extends downwardly beyond theelectrode bases 122 and the comb-like electrodes 122 a, 122 b of theinner frame 120 that serves as the moving part, as well as beyond thecomb-like electrodes 132 a, 132 b formed in the outer frame 130, by apredetermined length.

Each of the torsion bars 150, as shown in FIG. 2, has a layeredstructure including an upper layer 151, a lower-layer 152 and aninsulating layer 160 between them. The upper layer 151 and the lowerlayer 152 are electrically separated by the insulating layer 160. Theupper layer 151 is connected to the inner frame main portion 121 and thefirst outer frame 131 whereas the lower layer 152 is connected to theelectrode bases 122 and the second outer frame 132.

According to the micro mirror unit X1 having a structure as describedabove, when the first outer frame 131 is grounded, the members made ofthe same silicon material as and formed integrally with the first outerframe 131, i.e. the upper layer 151 of the torsion bars 150, the innerframe main portion 121, the torsion bars 140 and the mirror-formedportion 110, provide an electrical path that grounds the comb-likeelectrodes 110 a, 110 b and the comb-like electrodes 121 a, 121 b. Underthis state, by giving a predetermined electric potential to thecomb-like electrode 122 a or the comb-like electrode 122 b therebygenerating a static electric force between the comb-like electrode 110 aand the comb-like electrode 122 a or between the comb-like electrode 110b and the comb-like electrode 122 b, it becomes possible to pivot themirror-formed portion 110 about the pivotal axis A1. Likewise, by givinga predetermined electric potential to the comb-like electrode 132 a orthe comb-like electrode 132 b thereby generating a static electric forcebetween the comb-like electrode 121 a and the comb-like electrode 132 aor between the comb-like electrode 121 b and the comb-like electrode 132b, it becomes possible to pivot the mirror-formed portion 110 about thepivotal axis A2. The second outer frame 132 is electrically divided byair gaps for example, so as to provide electrical paths necessary forselectively giving the electric potential to the comb-like electrodes122 a, 122 b, 132 a, and 132 b.

FIG. 5 shows the micro mirror unit X1 mounted on a wiring substrate 400.The micro mirror unit X1 is shown in a sectional view taken in lines V—Vin FIG. 1. According to the micro mirror unit X1, the outer frame 130 isthicker than the moving part which includes the mirror-formed portion110 and the inner frame 120. Specifically, the second outer frame 132 ofthe outer frame 130 extends downwardly beyond the electrode bases 122and the comb-like electrodes 122 a, 122 b of the inner frame 120, aswell as beyond the comb-like electrodes 132 a, 132 b formed in the outerframe 130, by a predetermined length. The downward extension of thesecond outer frame 132 is beyond a depth reached by the moving part inoperation, e.g. a depth reached by the electrode bases 122 of the innerframe 120. With this arrangement, a space is provided for the movingpart to move under the state in which the wiring substrate 400 is bondedonto the bottom surface of the second outer frame 132, avoiding anunwanted contact of the moving part to the wiring substrate 400.Therefore, when the micro mirror unit X1 is mounted onto the wiringsubstrate 400, there is no need for placing a spacer between the micromirror unit X1 and the wiring substrate 400.

FIG. 6 through FIG. 8 show a first method of making the micro mirrorunit X1. This is a method for manufacturing the above-described micromirror unit X1 by way of micro-machining technology. For the sake ofsimplification of the drawings, each of the FIG. 6 through FIG. 8 givesonly one sectional view to show how formation is made for amirror-formed portion M, torsion bars T, inner frame F1, a set ofcomb-like electrodes E1, E2, and an outer frame F2. In effect, each ofthese sectional views provides a model that shows different sections ofthe material substrate to which micro machining is made. Specifically,the mirror-formed portion M represents a fragmentary section of themirror-formed portion 110, the torsion bars T represents a cross sectionof the torsion bars 140 or a fragmentary section of the torsion bars150, the inner frame F1 represents a fragmentary cross section of theinner frame 120 including the inner frame main portion 121 and theelectrode bases 122, the comb-like electrodes E1 represents afragmentary cross section of the comb-like electrodes 110 a, 110 b orthe comb-like electrodes 121 a, 121 b, the comb-like electrodes E2represents a fragmentary cross section of the comb-like electrodes 122a, 122 b or the comb-like electrodes 132 a, 132 b, and the outer frameF2 represents a fragmentary section of the outer frame 130 including thefirst outer frame 131 and the second outer frame 132.

In the manufacture of the micro mirror unit X1, first, as shown in FIG.6A, a substrate is prepared. The substrate is provided by an SOI(Silicon on Insulator) wafer 1. The SOI wafer 1 has a layered structureincluding a relatively thin first silicon layer 11, a relatively thicksecond silicon layer 12, and an insulating layer 160 which is anintermediate layer sandwiched between them. The first silicon layer 11is provided by an electrically conductive silicon doped with an n-typeimpurity such as P and As. The second silicon layer 12 is provided by anelectrically conductive silicon or poly-silicon doped with an n-typeimpurity such as P and As. Alternatively, these materials may be givenelectrical conductivity with a p-type impurity such as B. The insulatinglayer 160 is provided by silicon oxide grown on a surface of the firstsilicon layer 11 or the second silicon layer 12 by way of a thermaloxidation method. Alternatively to the thermal oxidation method, theinsulating layer 160 may be formed by using a CVD method. After theformation of the insulating layer 160, the first silicon layer 11 andthe second silicon layer 12 are bonded together, with the insulatinglayer 160 in between, whereby the SOI wafer 1 is completed. According tothe present embodiment, the first silicon layer 11 has a thickness of100 μm, the second silicon layer 12 has a thickness of 200 μm, and theinsulating layer 160 has a thickness of 1 μm.

Next, as shown in FIG. 6B, an oxide film pattern 51 is formed on thefirst silicon layer 11, and an oxide film pattern 52 is formed on thesecond silicon layer 12. Specifically, first, a CVD method is used forgrowing a film of silicon oxide on the first silicon layer 11 and on thesecond silicon layer 12. Then, the oxide films are etched via respectivepredetermined masks. A usable etching solution in this patterning stepis, for example, buffered hydrofluoric acid containing hydrofluoric acidand ammonium fluoride. It should be noted that oxide film patternformations in later steps can also be performed by using such a processas described here. The oxide film pattern 51 is to mask regions tobecome the mirror-formed portion M, the inner frame F1, the comb-likeelectrodes E1, and the outer frame F2 on the first silicon layer 11.More specifically, the oxide film pattern 51 is formed correspondinglyto a plan-view layout of the mirror-formed portion 110, the inner framemain portion 121, the comb-like electrodes 110 a, 110 b, the comb-likeelectrodes 121 a, 121 b, and the first outer frame 131 shown in FIG. 1.The oxide film pattern 52 is to mask regions to become the outer frameF2 on the second silicon layer 12. More specifically, the oxide filmpattern 52 is formed correspondingly to a plan-view layout of the secondouter frame 132 shown in FIG. 1.

Next, as shown in FIG. 6C, a resist pattern 53 is formed on the firstsilicon layer 11. Specifically, a liquid photo resist is applied bymeans of spin-coating to form a film on the first silicon layer 11. Thefilm is then exposed and developed to become the resist pattern 53. Thephoto resist usable in this step includes, for example, AZP4210(manufactured by Clariant Japan) and AZ1500 (manufactured by ClariantJapan). It should be noted that resist pattern formations performed inlater steps can also be made by such a process as described here, ofphoto resist film formation, exposure and development. The resistpattern 53 is to mask regions to become the mirror-formed portion M, thetorsion bars T, the inner frame F1, the comb-like electrodes E1, and theouter frame F2 on the first silicon layer 11. More specifically, theresist pattern 53 is formed correspondingly to a plan-view layout of themirror-formed portion 110, the torsion bars 140, 150, the inner framemain portion 121, the comb-like electrodes 110 a, 10 b, the comb-likeelectrodes 121 a, 121 b, and the first outer frame 131 shown in FIG. 1.

Next, as shown in FIG. 6D, the first silicon layer 11 masked by theresist pattern 53 is etched by means of DRIE (Deep Reactive Ion Etching)to a depth equal to the thickness of the torsion bars T. In the presentembodiment, this depth is 5 μm. During the DRIE, when performing theBosch process in which etching is alternated with sidewall protection,the etching with SF₆ gas is performed for about 8 seconds, which is thenfollowed by the sidewall protection with C₄F₈ gas performed for about6.5 seconds, with a bias power applied to the wafer being about 23 W.These conditions allow sufficient etching. The same conditions can alsobe used for DRIE processes performed in later steps.

Next, as shown in FIG. 7A, the resist pattern 53 is removed. Theremoving solution can be provided by AZ remover 700 (manufactured byClariant Japan). This can also be used for removal of resist patternsperformed in later steps.

Next, as shown in FIG. 7B, using the DRIE, the first silicon layer 11masked by the oxide film pattern 51 is etched until the insulating layer160 is reached. This step gives form to the mirror-formed portion M, thetorsion bars T, part of the inner frame F1, the comb-like electrode E1and part of the outer frame F2.

Next, as shown in FIG. 7C, a resist pattern 54 is formed on the secondsilicon layer 12. The resist pattern 54 is to mask the inner frame F1and the comb-like electrode E2 on the second silicon layer 12. Morespecifically, the resist pattern 54 is formed correspondingly to theplan-view layout of the electrode bases 122, the comb-like electrodes122 a, 122 b, and the comb-like electrodes 132 a, 132 b shown in FIG. 1.

Next, as shown in FIG. 7D, the second silicon layer 12 masked by theoxide film pattern 52 and the resist pattern 54 is etched by means ofDRIE, to a depth equal to the thickness of the comb-like electrode E2.

Next, as shown in FIG. 8A, the resist pattern 54 is removed. Then, asshown in FIG. 8B, the second silicon layer 12 masked by the oxide filmpattern 52 is etched until the insulating layer 160 is reached. Thisgives form to part of the inner frame F1, the comb-like electrode E2 andpart of the outer frame F2.

Next, as shown in FIG. 8C, by soaking into an etching solution, theexposed insulation layer 160 is removed by etching. During this step,the oxide film patterns 51, 52 exposed on the surface of the element areremoved at the same time. This step gives form to the mirror-formedportion M, the torsion bars T, the inner frame F1, and the comb-likeelectrodes E1, E2 within 100 μm from the insulating layer 160, and tothe outer frame F2 which includes the second outer frame 132 having athickness of 200 μm. This is how the micro mirror unit X1 ismanufactured.

According to such a method as described, the moving part and thetwo-step comb-like structure are thinner than the material substrateused, i.e. thinner than the wafer. Therefore, it becomes possible,regardless of the thickness to be given to the moving part and thetwo-step comb-like structure, to use a wafer that have a thicknesscapable of retaining sufficient strength throughout the entiremanufacturing process of the micro mirror unit. Now that it becomespossible to use a wafer that have a thickness capable of retainingsufficient strength regardless of the thickness to be given to themoving part and the two-step comb-like structure, the limitation to thesize of the flat surface of the wafer is reduced.

FIG. 9 and FIG. 10 show a second method of making the micro mirror unitX1. This also is a method for manufacturing the above-described micromirror unit X1 by way of micro-machining technology. For the sake ofsimplification of the drawings as used in FIG. 6 through FIG. 8, each ofthe FIG. 9 and FIG. 10 gives only one sectional view to show howformation is made for a mirror-formed portion M, torsion bars T, innerframe F1, a set of comb-like electrodes E1, E2, and an outer frame F2.

In the second method of manufacture, first, the same steps as describedfor the first method with reference to FIG. 6A through FIG. 6D and FIG.7A through FIG. 7C are followed, until the SOI wafer 1 is as shown inFIG. 9A. Specifically, in the SOI wafer 1 shown in FIG. 9A, the firstsilicon layer 11 masked by the oxide film pattern 51 is etched by meansof the DRIE, and the oxide film pattern 52 and the resist pattern 54 areformed on the second silicon layer 12.

Next, as shown in FIG. 9B, the first silicon layer 11 masked by theresist pattern 54 and the oxide film pattern 52 is etched by means ofDRIE until the insulating layer 160 is reached. Thereafter, as shown inFIG. 9C, the resist pattern 54 is removed.

Next, as shown in FIG. 9D, a spray is made from below as in the figureto form a resist pattern 55′. The photo resist solution used in thespraying can be provided by AZP4210 (manufactured by Clariant Japan)diluted to four times with AZ5200 thinner (manufactured by ClariantJapan).

Next, the photo resist 55′ is exposed and developed to form a photoresist 55 as shown in FIG. 10A. The resist pattern 55 is primarily toprotect the insulating layer 160.

Next, as shown in FIG. 10B, using the DRIE, the second silicon layer 12masked by the oxide film pattern 52 is etched to a predetermined depth.This step gives form to part of the inner frame F1 and the comb-likeelectrode E2.

Next, as shown in FIG. 10C, the resist pattern 55 is removed. Then, asshown in FIG. 10D, by soaking into an etching solution, the exposedinsulation layer 160 is removed by etching. During this step, the oxidefilm patterns 51, 52 exposed on the surface of the element are removedat the same time. This step gives form to the mirror-formed portion M,the torsion bars T, the inner frame F1, and the comb-like electrodes E1,E2 within 100 μm from the insulating layer 160, and to the outer frameF2 which includes the second outer frame 132 having a thickness of 200μm. This is how the micro mirror unit X1 is manufactured.

According to such a method as described, the moving part and thetwo-step comb-like structure are thinner than the material substrateused, i.e. thinner than the wafer. Therefore, the second method offersthe same advantages as achieved by the first method.

FIG. 11 and FIG. 12 show a third method of making the micro mirror unitX1. This also is a method for manufacturing the above-described micromirror unit X1 by way of micro-machining technology. For the sake ofsimplification of the drawings as used in FIG. 6 through FIG. 8, each ofthe FIG. 11 and FIG. 12 gives only one sectional view to show howformation is made for a mirror-formed portion M, torsion bars T, innerframe F1, a set of comb-like electrodes E1, E2, and an outer frame F2.

According to the third method, first, as shown in FIG. 11A, a substrateis prepared. The substrate is provided by an SOI (Silicon on Insulator)wafer 2. The SOI wafer 2 has a layered structure including a firstsilicon layer 13, a second silicon layer 14, and an insulating layer 160which is an intermediate layer sandwiched between them. According to thepresent embodiment, the first silicon layer 13 has a thickness of 100μm, the second silicon layer 14 has a thickness of 100 μm, and theinsulating layer 160 has a thickness of 1 μm. During the preparation ofthe SOI wafer 2, the silicon layers are given electrical conductivityand the insulating layer 160 is formed, in the same way as described forthe first method.

Next, as shown in FIG. 11B, an oxide film pattern 56 is formed on thefirst silicon layer 13, and an oxide film pattern 57 is formed on thesecond silicon layer 14. The oxide film pattern 56 is to mask regions tobecome the mirror-formed portion M, the inner frame F1, the comb-likeelectrodes E1, and the outer frame F2 on the first silicon layer 13.More specifically, the oxide film pattern 56 is formed correspondinglyto a plan-view layout of the mirror-formed portion 110, the inner framemain portion 121, the comb-like electrodes 110 a, 110 b, the comb-likeelectrodes 121 a, 121 b, and the first outer frame 131 shown in FIG. 1.The oxide film pattern 57 is to mask regions to become the inner frameF1 and the comb-like electrode E2 on the second silicon layer 14. Morespecifically, the oxide film pattern 57 is formed correspondingly to aplan-view layout of the electrode bases 122, the comb-like electrodes122 a, 122 b, and the comb-like electrodes 132 a, 132 b shown in FIG. 1.

Next, as shown in FIG. 11C, the third silicon layer 15 is bondeddirectly to the second silicon layer 14 of the SOI wafer 2. The thirdsilicon layer 15 is made of electrically conductive silicon doped withan impurity, and has a thickness of 100 μm. Further, the third siliconlayer 15 is formed with a relief space by means of DRIE at a locationcorresponding to the oxide film pattern 57. According to the presentembodiment, the relief space has a depth of 5 μm. The bonding in thisstep is performed under a vacuum of 10⁻⁴ Torr, and a temperature of1100° C. The bonding integrates the third silicon layer 15 with thesecond silicon layer 14.

Next, as shown in FIG. 11D, the first silicon layer 13 masked by theoxide film pattern 56 is etched by means of DRIE until the insulatinglayer 160 is reached. This step gives form to the mirror-formed portionM, the torsion bars T, part of the inner frame F1, the comb-likeelectrode E1 and part of the outer frame F2.

Next, as shown in FIG. 12A, an oxide film pattern 58 is formed on thethird silicon layer 15. The oxide film pattern 58 is to mask a region tobecome the outer frame F2. More specifically, the oxide film pattern 58is formed correspondingly to a plan-view layout of the second outerframe 132 shown in FIG. 1.

Next, as shown in FIG. 12B, the third silicon layer 15 masked by theoxide film pattern 58 is etched by means of DRIE until the oxide filmpattern 57 is exposed.

Next, as shown in FIG. 12C, the second silicon layer 14 masked by theoxide film pattern 57 and the oxide film pattern 58 is etched by meansof DRIE, until the insulating layer 160 is reached. This gives form topart of the inner frame F1, the comb-like electrode E2 and part of theouter frame F2.

Next, as shown in FIG. 12D, by soaking into an etching solution, theexposed insulation layer 160 is removed by etching. During this step,the oxide film patterns 56, 57, 58 exposed on the surface of the elementare removed at the same time. This step gives form to the mirror-formedportion M, the torsion bars T, the inner frame F1, and the comb-likeelectrodes E1, E2 within 100 μm from the insulating layer 160, and tothe outer frame F2 which includes the second outer frame 132 having athickness of 200 μm. This is how the micro mirror unit X1 ismanufactured.

According to such a method as described, it is possible to form themoving part and the two-step comb-like structure in a materialsubstrate, or a wafer, which is thicker than these members. Therefore,the third method offers the same advantages as achieved by the firstmethod. Before the step shown in FIG. 11D, no forming operation whichdecreases strength of the wafer is performed to the silicon layers.Thus, the size of the flat surface of the wafer is not excessivelylimited before the step shown in FIG. 11D.

FIG. 13 and FIG. 14 show a fourth method of making the micro mirror unitX1. This also is a method for manufacturing the above-described micromirror unit X1 by way of micro-machining technology. For the sake ofsimplification of the drawings as used in FIG. 6 through FIG. 8, each ofthe FIG. 13 and FIG. 14 gives only one sectional view to show howformation is made for a mirror-formed portion M, torsion bars T, innerframe F1, a set of comb-like electrodes E1, E2, and an outer frame F2.

According to the fourth method, first, as shown in FIG. 13A, a substrateis prepared. The substrate is provided by an SOI wafer 3. The SOI wafer3 has a layered structure including a first silicon layer 16, a secondsilicon layer 17, and an insulating layer 160 which is an intermediatelayer sandwiched between them. The second silicon layer 17 is alreadyshaped to correspond to the comb-like electrode E2 by means of DRIE. Thesecond silicon layer 17 is bonded to the first silicon layer 16 formedwith the insulating layer 160. The comb-like electrode E2 contacts theinsulating layer 160. According to the present embodiment, the firstsilicon layer 16 has a thickness of 100 μm, the second silicon layer 17has a thickness of 200 μm, and the insulating layer 160 has a thicknessof 1 μm. During the preparation of the SOI wafer 3, the silicon layersare given electrical conductivity and the insulating layer 160 isformed, in the same way as described for the first method.

Next, as shown in FIG. 13B, an oxide film pattern 59 is formed on thefirst silicon layer 16, and an oxide film pattern 60 is formed on thesecond silicon layer 17. The oxide film pattern 59 is to mask regions tobecome the mirror-formed portion M, the inner frame F1, the comb-likeelectrodes E1, and the outer frame F2 on the first silicon layer 16.More specifically, the oxide film pattern 59 is formed correspondinglyto a plan-view layout of the mirror-formed portion 110, the inner framemain portion 121, the comb-like electrodes 110 a, 110 b, the comb-likeelectrodes 121 a, 121 b, and the first outer frame 131 shown in FIG. 1.The oxide film pattern 60 is to mask regions to become the outer frameF2 on the second silicon layer 17. More specifically, the oxide filmpattern 60 is formed correspondingly to a plan-view layout of the secondouter frame 132 shown in FIG. 1.

Next, the same steps as described in the first method with reference toFIG. 6A through FIG. 6D and FIG. 7A through FIG. 7B are followed, untilthe SOI wafer 3 is as shown in FIG. 13C.

Next, as shown in FIG. 13D, a resist pattern 61 is formed on the secondsilicon layer 17. The resist pattern 61 is to mask regions to become theinner frame F1, the comb-like electrodes E2, and the outer frame F2 onthe second silicon layer 17.

Next, as shown in FIG. 14A, the second silicon layer 17 masked by theresist pattern 61 is etched by means of DRIE to a predetermined depth,or to the height of the comb-like electrode E2. Then, as shown in FIG.14B, the resist pattern 61 is removed.

Next, as shown in FIG. 14C, the second silicon layer 17 masked by theoxide film pattern 60 is etched by means of DRIE until the insulatinglayer 160 is reached. This step gives form to the part of the innerframe F1, the comb-like electrode E2 and part of the outer frame F2.

Next, as shown in FIG. 14D, by soaking into an etching solution, theexposed insulation layer 160 is removed by etching. During this step,the oxide film patterns 59, 60 exposed on the surface of the element areremoved at the same time. This step gives form to the mirror-formedportion M, the torsion bars T, the inner frame F1, and the comb-likeelectrodes E1, E2 within 100 μm from the insulating layer 160, and tothe outer frame F2 which includes the second outer frame 132 having athickness of 200 μm. This is how the micro mirror unit X1 ismanufactured.

According to such a method as described, it is possible to form themoving part and the two-step comb-like structure which are thinner thana material substrate used, i.e. a wafer. Therefore, the fourth methodalso offers the same advantages as achieved by the first method.

FIG. 15 is a perspective view of a micro mirror unit X2 according to thesecond embodiment of the present invention. FIG. 16 is a sectional viewtaken in lines XVI—XVI in FIG. 15. The micro mirror unit X2 includes amirror-formed portion 110, an inner frame 120 surrounding it, an outerframe 130′ surrounding the inner frame 120, a pair of torsion bars 140connecting the mirror-formed portion 110 with the inner frame 120 and apair of torsion bars 150 connecting the inner frame 120 with the outerframe 130′. The micro mirror unit X2 differs from the micro mirror unitX1 in the construction of the outer frame, but the mirror-formed portion110, the inner frame 120 and the torsion bars 140, 150 of the micromirror unit X2 are the same as those described for the micro mirror unitX1.

As shown clearly in FIG. 16, the outer frame 130′ has a layeredstructure including a first outer frame 131′, a second outer frame 132and an insulating layer 160 between them. The first outer frame 131′ andthe second outer frame 132 are electrically insulated from each other bythe insulating layer 160. As clearly shown in FIG. 16, the first outerframe 131′ extends upwardly beyond the inner frame main portion 121which is part of the moving part provided by the mirror-formed portion110 and the inner frame 120. The second outer frame 132 has the samestructure as described for the first embodiment.

FIG. 17 shows the micro mirror unit X2 mounted on a wiring substrate 400and covered by a transparent cover 401. In the figure, the micro mirrorunit X2 is shown in a section taken on lines XVII—XVII in FIG. 15.According to the micro mirror unit X2, the outer frame 130′ is thickerthan the moving part provided by the mirror-formed portion 110 and theinner frame 120. Specifically, the second outer frame 132 extendsdownwardly beyond the electrode bases 122 and the comb-like electrodes122 a, 122 b of the inner frame 120, and beyond the comb-like electrodes132 a, 132 b formed in the outer frame 130. The downward extension ofthe second outer frame 132 is beyond a depth reached by the moving partin operation, e.g. a depth reached by the electrode bases 122 of theinner frame 120. With this arrangement, a space is provided for themoving part to move under the state in which the wiring substrate 400 isbonded onto the bottom surface of the second outer frame 132, avoidingan unwanted contact of the moving part to the wiring substrate 400.Further, the first outer frame 131′ extends upwardly beyond themirror-formed portion 110, the comb-like electrodes 110 a, 110 b, theinner frame main portion 121 and the comb-like electrodes 121 a, 121 bof the inner frame 120. The downward extension of the first outer frame131′ is beyond a height reached by the moving part in operation, e.g. aheight reached by the comb-like electrodes 121 a, 121 b of the innerframe 120. With this arrangement, a space is provided for the movingpart to move under the state in which the transparent cover 401 isbonded onto the upper surface of the first outer frame 131′, avoiding anunwanted contact of the moving part to the transparent cover 401. Thus,according to the micro mirror unit X2, since the first outer frame 131′and the second outer frame 132 extend beyond the moving part, there isno need for placing a spacer between the micro mirror unit X2 and thewiring substrate 400 or the transparent cover 401 when the micro mirrorunit X1 is mounted onto the wiring substrate 400.

FIG. 18 through FIG. 20 show a method of making the micro mirror unitX2. This is a method for manufacturing the above-described micro mirrorunit X2 by way of micro-machining technology. For the sake ofsimplification of the drawings as used in FIG. 6 through FIG. 8, each ofthe FIG. 18 through FIG. 20 gives only one sectional view to show howformation is made for a mirror-formed portion M, torsion bars T, innerframe F1, an inner frame F1, a set of comb-like electrodes E1, E2, andan outer frame F2.

In the manufacture of the micro mirror unit X2, first, as shown in FIG.18A, a substrate is prepared. The substrate is provided by an SOI wafer4. The SOI wafer 4 has a layered structure including a first siliconlayer 18, a second silicon layer 19, and an insulating layer 160 whichis an intermediate layer sandwiched between them. The first siliconlayer 18 is already formed the torsion bars T therein. Specifically, thetorsion bars T can be formed in the first silicon layer 18 by firstforming a predetermined groove in the first silicon layer 18, thenforming an oxide film on the groove surface, and then filling the groovewith poly-silicon. The first silicon layer 18 structured as such isbonded to the second silicon layer 19 formed with the insulating layer160, with the torsion bars T contacted to the insulating layer 160.According to the present embodiment, the first silicon layer 18 has athickness of 100 μm, the second silicon layer 19 has a thickness of 100μm, and the insulating layer 160 has a thickness of 1 μm. The torsionbars have a thickness of 5 μm. During the preparation of the SOI wafer4, the silicon layers are given electrical conductivity and theinsulating layer 160 is formed in the same way as described for thefirst method.

Next, as shown in FIG. 18B, an oxide film pattern 62 is formed on thefirst silicon layer 18, and an oxide film pattern 63 is formed on thesecond silicon layer 19. The oxide film pattern 62 is to mask regions tobecome the mirror-formed portion M, the inner frame F1, and thecomb-like electrodes E1 on the first silicon layer 18. Morespecifically, the oxide film pattern 62 is formed correspondingly to aplan-view layout of the mirror-formed portion 110, the inner frame mainportion 121, the comb-like electrodes 110 a, 110 b, and the comb-likeelectrodes 121 a, 121 b shown in FIG. 1. The oxide film pattern 63 is tomask regions to become the outer frame F2 and the comb-like electrode E2on the second silicon layer 19. More specifically, the oxide filmpattern 63 is formed correspondingly to a plan-view layout of theelectrode bases 122, the comb-like electrodes 122 a, 122 b and thesecond outer frame 132, 132 b, shown in FIG. 1.

Next, as shown in FIG. 18C, the first silicon layer 18 of the SOI wafer4 is bonded directly to a third silicon layer 20. Further, a fourthsilicon layer 21 is bonded directly to the second silicon layer 19. Thethird silicon layer 20 and the fourth silicon layer 21 are each made ofelectrically conductive silicon doped with an impurity, and has athickness of 100 μm. Further, the third silicon layer 20 and the fourthsilicon layer 21 is formed with relief spaces in advance by means ofDRIE at locations corresponding to the oxide film patterns 62, 63.According to the present embodiment, the relief spaces have a depth of 5μ. The bonding in this step is performed under a vacuum of 10⁻⁴ Torr,and a temperature of 1100° C. The bonding integrates the third siliconlayer 20 with the first silicon layer 18, and the fourth silicon layer21 with the second silicon layer 19.

Next, as shown in FIG. 19A, an oxide film pattern 64 is formed on thethird silicon layer 20, and an oxide film pattern 65 is formed on thefourth silicon layer 21. The oxide film pattern 64 is to mask regions tobecome the outer frame F2 on the third silicon layer 20 and the firstsilicon layer 18. More specifically, the oxide film pattern 64 is formedcorrespondingly to a plan-view layout of the first outer frame 131′shown in FIG. 15. The oxide film pattern 65 is to mask a region tobecome the outer frame F2 on the fourth silicon layer 21. Morespecifically, the oxide film pattern 65 is formed correspondingly to aplan-view layout of the second outer frame 132 shown in FIG. 15.

Next, as shown in FIG. 19B, the third silicon layer 20 masked by theoxide film pattern 64 is etched by means of DRIE until the oxide filmpattern 62 is exposed. Next, as shown in FIG. 19C, the first siliconlayer 18 masked by the oxide film pattern 62 and the oxide film pattern64 is etched by means of DRIE, until the insulating layer 160 isreached.

Next, as shown in FIG. 20A, the fourth silicon layer 21 masked by theoxide film pattern 65 is etched by means of DRIE until the oxide filmpattern 63 is exposed. Next, as shown in FIG. 20B, the second siliconlayer 19 masked by the oxide film pattern 63 and the oxide film pattern65 is etched by means of DRIE, until the insulating layer 160 isreached.

Next, as shown in FIG. 20C, by soaking into an etching solution, theexposed insulation layer 160 is removed by etching. During this step,the oxide film patterns 62 through 65 exposed on the surface of theelement are removed at the same time. This step gives form to themirror-formed portion M, the torsion bars T, the inner frame F1, and thecomb-like electrodes E1, E2 within 100 μm from the insulating layer 160,and to the outer frame F2 including the first outer frame 131′ and thesecond outer frame 132 having a thickness of 200 μm. This is how themicro mirror unit X2 is manufactured.

According to such a method as described, it is possible to form themoving part and the two-step comb-like structure in a materialsubstrate, i.e. a wafer, which is thicker than these members. Therefore,this method also offers the same advantages as achieved by the firstmethod. Before the step shown in FIG. 19B, no forming operation whichdecreases strength of the wafer is performed to the silicon layers.Thus, the size of the flat surface of the wafer is not excessivelylimited before the step shown in FIG. 19B.

In any of the methods for making the micro mirror units described above,formation of the mirror surface 111 on the mirror-formed portion 110 isperformed before the region to become the mirror-formed portion 110 iscovered by the oxide film pattern by means of CVD method. The mirrorsurface 111 can be formed by spattering Au or Cr onto a region to becomethe mirror-formed portion 110 on the silicon layer.

As for the process performed to the lower layer of the insulating layer160 in the micro mirror unit X2, the process described earlier may bereplaced by one of the processes performed to the lower layer in one ofthe first and the fourth methods described earlier. Such a combinationof processes also makes possible to manufacture a micro mirror unit X2having its outer frame 130′ extending both upwardly and downwardly.

The present invention being thus described, it is obvious that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention, and allsuch modifications as would be obvious to those skilled in the art areintended to be included within the scope of the following claims.

1. A micro mirror unit comprising a moving part, a frame and a torsionbar connecting the moving part to the frame, wherein the moving part,the frame and the torsion bar are formed integral from a common materialsubstrate having a layered structure including an intermediate layer andsilicon layers sandwiching the intermediate layer, wherein the movingpart includes: a first intermediate portion originating from theintermediate layer; a first structural member held in contact with thefirst intermediate portion and provided with a mirror portion; and asecond structural member held in contact with the first intermediateportion on a side opposite to the first structural member; wherein theframe includes: a second intermediate portion originating from theintermediate layer; a third structural member held in contact with thesecond intermediate portion on a same side as the first structuralmember; and a fourth structural member held in contact with the secondintermediate portion on a same side as the second structural member; andwherein the third structural member extends beyond the first structuralmember in a layering direction of the layered structure.
 2. The micromirror unit according to claim 1, wherein the moving part includes afirst comb-like electrode, the frame including a second comb-likeelectrode for operation of the moving part by static electric forcegenerated between the first and the second comb-like electrodes.
 3. Themicro mirror unit according to claim 2, wherein the first comb-likeelectrode is formed in the first structural member, the second comb-likeelectrode being formed in the fourth structural member at a portioncontacting the second intermediate portion.
 4. The micro mirror unitaccording to claim 1, wherein the moving part includes: a relay frameconnected to the frame via the torsion bar; a mirror-formed portionspaced from the relay frame; and a relay bar connecting the relay frameto the mirror-formed portion, the relay bar extending in a directionacross a direction in which the torsion bar extends.
 5. The micro mirrorunit according to claim 4, wherein the mirror-formed portion includes athird comb-like electrode, the relay frame including a fourth comb-likeelectrode for operation of the mirror-formed portion by static electricforce generated between the third and the fourth comb-like electrodes.6. The micro mirror unit according to claim 5, wherein the thirdcomb-like electrode is formed in the first structural member, the fourthcomb-like electrode being formed in the second structural member.